This invention relates to the field of devices and methods for charging lithium-ion cells (or batteries) and specifically to a charging circuit including a power transformer in which the loading curve of the power transformer is used to limit the current flow to the lithium-ion cell (or battery) and a method for charging lithium-ion cells (or batteries) in which the loading curve of a power transformer is used to limit the current flow to the lithium-ion cell (or battery).
Lithium-ion cells are used in battery packs where high energy density and low weight are required. However, lithium-ion cells can be dangerous if operated outside of their rated specifications. Typically, such batteries are used in controlled environments and are accompanied by suitable protective devices to prevent such problems as short circuits, unduly high temperatures and over-discharge. A number of such protective devices are typically installed in the battery pack. It is standard industry practice that lithium-ion cells are equipped with in-pack circuitry that provides the necessary protection for the cell in use. Although the in-pack circuitry will provide over-all protection, suitable cell charging circuitry is required to provide repeated charging of the cell while satisfying applicable charging and operational constraints that vary somewhat from one cell type to another, as the manufacturer may have specified for any given design.
Particularly, lithium-ion cells carry a risk of generating excess gas due to overcharge or overdischargexe2x80x94this may cause the safety vent of the battery pack Lo open and release electrolyte into the atmosphere. If this release of electrolyte is continued, the cells can lose sufficient electrolyte that they are disabled. Further, overcharge or overdischarge may generate excess heat, causing a severe rise in temperature that can reduce the ability of the cell to retain energy and reduce the number of charging cycles the cell can undergo before it must be replaced. More seriously, overcharging or overdischarging may occur to such an extent that the lithium metal is isolated from the other elements and may become plated onto one of the electrodes. Lithium metal is explosive in water and will, in varying degrees, react with the moisture in the atmosphere. Lithium-containing batteries have been known to catch fire, although more recent safety designs have reduced the chances of this occurrence. The avoidance of overcharge voltage and overcharge current during charging of a lithium-ion cell is therefore an important objective in the use of lithium-ion cells, has been achieved by a number of known regulator circuits, and is also a principal objective of the present invention.
It is known that the attained charge capacity of a lithium-ion cell is significantly reduced if the charging voltage is less than the manufacturer""s recommended maximum charging voltage (say 4.1 volts). With a drop of charging voltage of only 0.05V (approximately 1%), a loss of up to 5% in charge capacity occurs. However, if the charging voltage reaches only 4.0 volts (a drop of 0.1V or approximately 2%) then a loss of charge capacity of up to 12% occurs. On the other hand, as pointed out previously, if one exceeds the manufacturer""s recommended maximum charging voltage, the life cycle of the cell is decreased, or worse, catastrophic breakdown of the cell can occur. Therefore one is compelled by these combined constraints to charge the lithium-ion cell at a voltage (at least at the end of the charging cycle) that is as close as reasonably possible to the maximum charging voltage without exceeding it.
Previous battery charging circuits for lithium-ion cells or batteries are known that include suitable regulator devices to maintain charging voltage and current within acceptable constraints. The xe2x80x9ccharge inhibition voltagexe2x80x9d refers to the value that the cell manufacturer has set as the upper limit of operating/charging voltage of the cell. If the voltage exceeds this value, lithium metal may become plated to an electrode, with potentially dire consequences as discussed above. The xe2x80x9cmaximum charging voltagexe2x80x9d is also established by the manufacturer at a lower value than the charge inhibition voltage; if for example the charge inhibition voltage is 4.35 volts for a representative cell, the maximum charging voltage is typically set at about 4.1 or 4.2 volts. Lithium-ion cell manufacturers have found that operation above the maximum charging voltage tends to reduce severely the recharging life cycle of the battery. Accordingly, in order to ensure that charging voltage is no greater than the set maximum charging voltage for the cell, controlled lithium-ion cell charging circuits typically provide a maximum output charge voltage that is no more than the maximum charging voltage.
In a typical charging circuit, an alternating current source operating at line voltage (typically 110-120 volts in North America) is applied to the primary winding of a transformer whose secondary winding applies a relatively low AC voltage to a bridge rectifier. The output of the bridge rectifier is applied across a smoothing capacitor to the load (the load in the charging circuit is the lithium-ion cell or battery to be charged). If no circuit elements were present other than the foregoing, the output voltage delivered to the lithium-ion cell would be at risk of exceeding the maximum charging voltage and ultimately might exceed the charge inhibition voltage of the lithium-ion cell. Accordingly, interposed between the bridge circuit and the lithium-ion cell or battery is a regulator circuit for limiting the voltage and current applied to the lithium-ion cell or battery during the charging operation.
A general purpose battery charger is described in U.S. Pat. No. 3,736,490 (Fallon et al.). This patent describes a battery charger incorporating a high leakage transformer and multiple rectifiers for regulating the charge current and the charge voltage applied to a battery. The high leakage transformer is used to provide impedance isolation between the input and the output circuit of the transformer and thus to protect the semiconductor components from line transients. The transformer is selected for maintaining a trickle charge current to the battery after a controlled rectifier providing supplemental current to the battery has been cut-off.
Another charging device defining the general state of the art is described in the abstract of Japanese Patent publication no 07296854 (Mitsui). The abstract describes a device for charging a battery that includes a constant current generator for charging the battery at a constant current at the initial stage of charging, and a constant voltage generator for performing constant voltage charging after a predetermined charging voltage has been reached.
Two types of regulator circuit are conventionally used, both of which are constant current/constant voltage regulator circuits, viz a linear regulator circuit, and a switching regulator circuit.
A switching regulator circuit includes a specially-designed charge control integrated circuit (IC) device for use with the other circuit elements. Such IC device is connected within the switching regulator circuit in constant-current mode. With the regulator operating in constant-current mode, charging continues at a constant current until the voltage across the lithium-ion cell or battery reaches the pre-set maximum charging voltage. The circuit then limits the output charging voltage to the maximum charging voltage, using a pulse-width modulation technique. According to this technique, the length of time that charge current is applied to the lithium-ion cell load during each AC. cycle is progressively and gradually decreased as charging proceeds.
The commercially available Benchmarq(trademark) model bq2054 IC device and the 4C(trademark) Technologies 4C-101656Li device are representative examples of charge control IC elements for use with a switching regulator circuit of the type described above.
As an alternative to the switching regulator, the principal other previously known regulated lithium-ion cell charging circuit includes a linear regulator incorporating a pair of suitable linear regulator charge control IC devices, one such device being connected within a charge current regulation subcircuit of the overall charging circuit, and the other within a charge voltage regulation subcircuit. These linear IC devices incorporate transistors constrained to operate within a relatively linear region of operation which happens to be a relatively inefficient region of operation. (By contrast, switching regulator IC devices permit the transistors in the integrated circuit to operate in relatively efficient Class C mode of operation.) Until fairly recently, such linear regulator circuits were considerably less efficient than switching regulator circuits, and generated an undesirable amount of heat, although such linear regulators were typically lower in cost than switching regulators. For the older type of linear regulator, the minimum differential voltage (generally referred to as the xe2x80x9cminimum dropout voltagexe2x80x9d) between unregulated voltage at the input of the linear regulator circuit and the regulated output charge voltage of the linear regulator circuit was approximately 1.5 volts when used for constant-voltage regulation and 1.2 volts when used for constant-current regulation. As this differential voltage is relatively high, leading to relatively inefficient charging, linear regulators using the older type of linear regulator IC device were typically used only for low-power charging requirements.
A previously known battery-charging circuit not designed specifically for lithium-ion cells or batteries that uses only a single linear regulator charge control IC device that provides both charge current regulation and voltage regulation is shown in FIG. 11-2 of Gordon McComb, Robot Builder""s Bonanza (New York, 1987), p. 81. However, that circuit includes a current limiting resistor and a silicon-controlled rectifier and appears to be designed to provide constant charging current until the charging voltage reaches the maximum charging voltage.
More recently, a new generation of linear regulator charge control IC devices has been developed that offers significant improvements in efficiency and a reduction in heat generation. These new regulators are frequently referred to as low drop-out voltage regulators or xe2x80x9cLDOxe2x80x9d regulators, because the minimum differential voltage (dropout voltage) between input supply voltage and output charge voltage can be as low as about 0.5 volts when used for constant-voltage regulation and as low as 1.2 volts (about the same as for the older type of linear regulator IC device) when used for constant-current regulation of the charging circuit. The 0.5-volt differential when the IC device is operated in constant-voltage mode permits these LDO regulators to operate from an unregulated DC supply voltage that is appreciably closer to the maximum charging voltage than was the case for the older linear regulator IC devices, thereby reducing power dissipation.
The older type of linear regulator charge control IC device is exemplified by the Motorola(trademark) LM317 IC device. The more recently available LDO linear regulator charge control IC device is exemplified by the Micrel(trademark) MIC29372 IC device.
Despite the improvements effected in IC design, lithium-ion cell charging circuits of the types previously known remain inherently inefficient because they operate from unregulated DC power that is supplied at a voltage significantly above the maximum charging voltage; the inefficiencies are also due to the conventional use of bath charge current and charge voltage regulating subcircuits, both of which dissipate energy.
Prior art, such as European Patent document EP 0 825 699 A (Benchmark Microelectronics), teaches that both charge current and charge voltage should be actively regulated during the charging of lithium-ion cell; the charge current initially at a constant value until the charge voltage reaches the manufacturer""s suggested maximum charging voltage and the charging voltage at the maximum charging voltage thereafter. Actively regulating current to a constant value requires that the regulating subcircuit be supplied with a high enough voltage that the regulator will not drop out of regulation as the charging voltage increases to the maximum charging voltage
The conventional design approach heretofore taken for the design of the regulation of lithium-ion cell or battery charging circuits proceeds on the premise that it is a good idea for the regulating circuit to be constantly active and to be regulating charging voltage and/or charging current throughout the complete cell charging process. (Herein frequent reference will be made to the xe2x80x9ccellxe2x80x9d to be charged, it being understood that with appropriate adjustments, one may in each case charge a battery of cells. Generally, a reference to a xe2x80x9cbatteryxe2x80x9d should be understood to include a reference to a single cell.)
According to the invention, the transformer used in the lithium-ion charging circuit is selected so that its inherent current-limiting characteristic (loading effect) permits the circuit to charge the lithium-ion cell during an initial period in which the regulator circuit need not perform any regulating function. This enables a satisfactory regulator circuit to be designed according to the invention using only a single charge control IC device that in an initial stage of the charging operation is in non-regulating mode, permitting the rectified transformer secondary output to be applied to the lithium-ion cell with only a minimum voltage drop across the single IC device (present in a voltage regulating subcircuit), as compared to two voltage drops across two IC devices (one in a current-regulating subcircuit and one in a voltage-regulating subcircuit) that would be present in conventional charging circuits, thereby affording substantial energy savings. When the charge voltage reaches a pre-set threshold level, the regulator circuit functions for the remainder or the charging operation in a manner similar to that of previous voltage regulation subcircuits, but with less overall power loss, since there is no separate current-regulation subcircuit present.
Accordingly, the invention provides a charging circuit for a lithium-ion cell (or battery) including a selected suitable transformer characterized by an inherent secondary output current-limiting capability that meets the initial current-limiting needs of the charging circuit, in combination with a suitable rectifier circuit (that may itself be of conventional design) and a linear charge-voltage regulating subcircuit that during the initial part of the charge cycle does not operate in regulating mode. Otherwise the linear charge-voltage regulator subcircuit and the rest of the circuit may be of conventional design, except that no separate charge-current regulator subcircuit is necessary nor present, thereby avoiding the associated power dissipation that occurs in such subcircuit present in conventional designs.
During the initial stage of the charging operation, charge voltage and charge current are maintained within acceptable limits by the condition of the discharged lithium-ion cell and the inherent secondary winding current-limiting characteristic of the transformer itself, and therefore the linear charge-voltage regulating subcircuit drops the supply voltage only by a minimum voltage drop (the minimum dropout voltage) between the rectified transformer secondary output and the lithium-ion cell being charged. The charge current applied during this initial stage slowly declines as the voltage across the cell being charged increases. For that reason, this initial mode of operation of the charging circuit may be referred to as xe2x80x9ctaper current modexe2x80x9d, since the current tapers off from an initial value varying more or less linearly with time to a reduced value.
During the later stage of the charging operation, the linear charge-voltage regulating subcircuit operates in the same manner as a conventional such subcircuit to limit applied charge voltage to the maximum charging voltage, during which time charge current decreases substantially logarithmically in the same manner as would occur in a conventional charging circuit incorporating linear regulation. Preferably the linear regulator IC device used in the charge-voltage regulating subcircuit is of the LDO type for maximum efficiency and charge capacity.
The inventor has found that the charge-current regulator subcircuit and the consequent power dissipation associated with such subcircuit may be eliminated without significantly affecting the performance of the battery charger while maintaining the charging voltage within safe limits. The elimination of the current-limiting subcircuit offers both improved energy efficiency and reduced cost of manufacture of the charging circuit, because not only is one subcircuit eliminated, but the required transformer can be smaller and lighter.
Note that it is important that the current rating and secondary voltage of the transformer be carefully selected, both to prevent damage to the cell during the initial charging stage and to provide an appropriate transformer loading curve so that the supply voltage begins to be regulated after the desired portion of the charging cycle has been completed. Specifically, a current rating for the transformer should be selected that is not greater than the maximum charging current for the cell or battery suggested by the manufacturer. The secondary voltage of the transformer (and therefore the characteristics of the transformer loading curve) should then be selected so that when the maximum charging current is flowing through the secondary winding of the transformer, the voltage supplied to the voltage regulating subcircuit is approximately equal to the sum of (1) a minimum charging voltage of the cell or battery to be charged selected to be somewhat less than the manufacturer""s nominal voltage rating of the battery and (2) the minimum voltage drop across the voltage regulating subcircuit. To compensate for line voltage variations, it is advisable to select the secondary voltage of the transformer based upon the maximum expected transformer primary voltage, rather than upon the average primary voltage, to avoid having the current flow during the initial charging stage exceed the transformer rating due to higher than average primary voltage.
A minimum charging voltage somewhat less than the nominal voltage is desirable, although the exact voltage used is not critical. For example, the battery manufacturer""s specifications for the battery for which the charge is being designed should provide the charging voltage as a function of time, assuming constant current until the charging voltage rises to the maximum charging voltage. In typical batteries known to the inventor, the charging voltage increases almost instantly from the discharged voltage (which may be as low as 2.5 volts) to approximately 3.3 to 3.7 volts reaching roughly 3.6 to 3.9 volts within a few minutes, depending upon a number of factors including the age and prior use of the battery. After the first few minutes the charging voltage continues to climb, but somewhat more slowly, until it reaches the maximum charging voltage of 4.1 or 4.2 volts as specified by the manufacturer (at which point the charging circuit must clamp the voltage or the battery may be damaged). While an initial charging voltage of 3.4 volts or even less could be used, the inventor has found that using an initial charging voltage of 3.5 to 3.6 volts to select the current rating of the transformer does not cause the charging current during the first few minutes under charge to reach levels high enough to adversely affect the battery being charged.
In accordance with the invention, for given battery specifications, the transformer selected for the charger will have a lower power rating (a lower current rating at the rated voltage) because the charging current decreases as the charging voltage increases. In a conventional charger in which current is regulated to a constant value until the charging voltage rises to the maximum charging voltage, the power consumed by the circuit must increase as the voltage rises as the current is being held constant. Hence the transformer must be rated to provide the maximum charging current at the maximum charging voltage, rather than at the minimum charging voltage selected as discussed above. A transformer with a lower power rating is lighter, smaller, and less expensive and generates less heat.
As mentioned, in this specification, in many passages, reference will be made to the charging of a lithium-ion cell; the representative voltages and currents specified at various points in the charging circuit are for a representative such cell, and the charging circuit parameters for such cell will be given typical values. However, it is to be understood that there is a variability in the characteristics of commercially-manufactured lithium-ion cells; such variability has to be taken into account in establishing various critical voltage and current values within the charging circuit. Further, it is to be understood that a given charging circuit could be designed to charge two or more lithium cells arranged in parallel or in series, and that depending upon the load for the circuit (i.e. the number of lithium-ion cells to be charged and whether they are connected in parallel or series) such values again would require adjustment from the typical values given in this specification.
The method according to the invention may be referred to as a xe2x80x9cstarved regulator techniquexe2x80x9d or as a xe2x80x9ctapered current/constant voltagexe2x80x9d technique. Reference to a xe2x80x9cstarved regulatorxe2x80x9d is appropriate because during the initial charging phase, the linear regulator IC device does not limit the charge voltage as the supply voltage is too low to require limiting. The regulator is starved for lack of voltage; this is not the way in which such regulators are designed to be used. The term xe2x80x9ctapered current/constant voltagexe2x80x9d is appropriate because current steadily diminishes as the threshold voltage is approached at which charge voltage regulation commences; charge voltage is maintained at a constant value during the regulated stage of the charging operation.
While the invention is optimized if the more recently available LDO charge-control IC device is used, the invention may also make use of the older generation of linear IC devices, and in that event entails advantages of the sort recited in the preceding description relative to previously known circuits that employ the older generation of linear IC devices. In each case, the conventional current-regulating subcircuit can be eliminated.